Template:SGC BRD3

BRD3: Human bromodomain containing protein 3 (domains 1 and 2)
PDB Code: 2nxb and 2oo1

2nxb: Filippakopoulos, P., Bullock, A., Cooper, C., Keates, K., Berridge, G., Pike, A., Bunkoczi, G., Gileadi, O., Lee, W.H., Muller, S., Marsden, B.D., Sundstrom, M., von Delft, F., Knapp, S.

2oo1: Filippakopoulos, P., Bullock, A., Papagrigoriou, E., Keates, T., Cooper, C., Smee, C., Ugochukwu, E., Debreczeni, J., Gileadi, O., von Delft, F., Lee, W.H., Muller, S., Marsden, B.D., Sundstrom, M., Knapp, S.

Description
BRD3 belongs to the BET subclass of proteins, which are distinguished by two N-terminal bromodomains and one ET (Extra Terminal) domain. BRDs have been found to be associated with chromatin. The poorly characterized ET domain functions as a protein-protein interaction motif and may be part of an atypical serine-kinase activity. The subclass consists of at least four members in mouse and human, Brd2 (also referred to as Fsrg1, RING3), Brd3(Fsrg2, ORFX), Brd4 (Fsrg4, MCAP/HUNK1), and Brdt (Fsrg3, BRD6). BRD proteins are related to the female sterile homeotic protein gene in Drosophila, a gene required maternally for proper expression of other homeotic genes, such as Ubx, which is involved in pattern formation. BRD3 mRNA is ubiquitously expressed in human adult and fetal tissues with highest expression in testis, ovary, placenta, uterus, and brain. BRD3 expression is induced in activated lymphocytes and it is highly expressed in undifferentiated ES cells whereas expression levels are reduced upon endothelial differentiation.

Down regulated expression or loss of BRD3 has been detected in biopsies of nasopharyngeal carcinomas and altered expression levels have been found in bladder cancer. In addition, the BRD3 gene is located to chromosome 9q34, a region susceptible to genomic rearrangement in tumors. BRD3 has also been proposed as a marker for hormone dependent cancer. BRD3 also interacts with LANA-1, the Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen 1, which is required for the replication of episomal viral genomes. Here we report the structure of the first and second individual bromodomains of BRD3.

Structural features
 The structures of BRD3 discussed here comprises two independent structures of the first bromo domain (BD1) including the sequence between residue Glu25 and Glu144 as well as the second bromo domain (BD2) (residues Lys307-Pro416). The asymmetric unit of BD1 contains two protein molecules in addition to one sodium ion and 7 ethylene glycole molecules used as a cryo protecting agent. Both BD1 molecules present in the asymmetric unit superimpose with an r.m.s.d. of ~1Å. The main structural differences are located in the large loop region connecting helix 1 with helix 2 (Asp64-Lys78) which is involved in the binding of acetylated lysine containing sequences. The N-terminal residues Glu25-Gly333 were only visible in chain A.

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The interface between both BRD3 BD1 monomers covers a large mainly hydrophobic surface area suggesting that this domain might be dimeric in solution. Dimerization of bromo domains has also been suggested for the closely related protein BRD2. However, the interface which is formed in BRD2 by the two C-terminal helices is not conserved in BRD3. As expected from their high degrees of sequence conservation (78% identity) the overall fold of BRD3 is closely related to the bromo domain of BRD2 (r.m.s.d 0.7 Å between chain B BRD3 and chain A BRD2; pdb-code: 1x0j).

The asymmetric unit of BRD3 BD2 contains 4 protein molecules, 2 sodium ions as well as a number of ethylene glycole molecules. Also in this case, the putative dimmer interface is different to the one observed in BRD2. The interface between BRD3 BD2 is mainly governed by a few hydrophilic interactions making it not likely that this molecule is dimeric in solution.

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